Intensity control for oscilloscope display



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Sept. 14, 1965 B. R. JONES INTENSITY CONTROL FOR OSCILLOSCOPE DISPLAYOriginal Filed Dec. 17, 1962 3 Sheets-Sheet 2 NMFTII.

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United States Patent O 3,206,633 INTENSITY CONTROL FOR OSCILLOSCOPEDISPLAY Billie R. Jones, Boston, Mass., assignor to the United States ofAmerica as represented by the Secretary of the Air Force Originalapplication Dec. 17, 1962, Ser. No. 245,327. Divided and thisapplication Dec. 3, 1963, Ser. N o.

The invention described herein may be manufactured and used by or forthe United States Government for governmental purposes without paymentto me of any royalty thereon.

This invention relates to display apparatus, and more particluarly tographic displays on the screen of a cathoderay oscilloscope forphotographic and recording purposes suitable to both still andcontinuously moving film cameras.

This patent application is a division of my pending patent applicationentitled Method and Apparatus for Oscilloscope Photography, bearingSerial Number 245,- 327, and tiled December 17, 1962.

l Thei oscilloscope is an instrument comprised of a cathode-ray tube andassociated circuits for use in viewing, photographing and recording waveshapes of voltages or currents. The cathode ray tube is comprised ofthree parts: an electron gun for supplying a stream of electrons in theform of a beam; deflection plates for changing the direction of theelectron beam; and a screen covered with a material which gives offlight when struck by the stream of electrons directed at it by the gun.There is also included an intensity modulation control which determinesthe amount of beam current. The intensity control feature of anoscilloscope is usually referred to as the Z axis.

The associated circuits in the oscilloscope include a time-basegenerator which generates an output signal utilized to move theelectronbeam from left to right side of the screen at a uniform rate.The beam is then returned hurriedly to the left side where it beginsanother sweep across the screen. This action is accomplished bygenerating a voltage that rises at a uniform rate to a certain value andthen quickly drops back to its starting value. A wave shape such as thisis called a saw-tooth wave.

The saw-tooth Voltage wave is applied to the horizontal deflectionplates, where it causes the electron stream to change direction. Sincepositive voltages repel and negative voltages attract electrons, thegradual rise in voltage causes the left plate to become increasinglynegative and the right plate increasingly positive and thereby causesthe spot to move across the screen. The quick drop of the voltage backto its starting value returns the spot from right to left in a veryshort time, called the iiy-back time.

The saw-tooth voltage is generated by the time base generator, and thisvoltage or some other desired external voltage for horizontal deflectionapplied at the terminals marked horizontal signal input, may then be fedinto a horizontal deflection ampliiier which increases the amplitude tothat needed for a trace of the desired length. The horizontal deectionportion of the oscilloscope is referred to as the H axis. The voltage orcurrent to be observed is applied to the vertical signal terminals andmay be ICC amplified to the desired value before being applied to thevertical deiiection plates. With voltages thus applied simultaneously tothe vertical plates and the horizontal plates, the deiiection of thebeam is the resultant of the two forces. The vertical deflection portionof the oscilloscope is known as the Y axis.

One of the limitations in photographing and recording the wave shapes onthe screen of an oscilloscope resides in dark background caused by theexposure of the iilm by the horizontal sweep trace. A furtherlimitationis the fading out of the leading and trailing edges of inputsignals which have a very fast rise time, such as square waves.

The object of my invention is to utilize the Z axis, or intensitymodulation, feature of an oscilloscope, in addition to the horizontaland vertical axes, to provide a display in which the trace of the signalto be photographed will be of a greater intensity than the horizontalsweep, thus improving measurement and analysis of transient signals.

The advantages of this type of display over the conventional type are,first, that when using a continuously moving film camera to recordtransients, such as contained in transmitted radio pulses, thehorizontal sweep may be eliminated and the signal will still be visible,thus eliminating the dark background caused by the exposure of the filmby the horizontal sweep allowing an easier and more accurate measurementand analysis of the signals; and second, the intensity modulation of theinput signal will eliminate the fading out of the leading and trailingedges of input signals which have a very fast rise time, such as squarewaves.

The features of this invention, which are believed to be new, are setforth with particularity in the appended claim. The invention itself,however, together with further objects and advantages thereof may bestbe understood by reference to the following description when taken inconjunction with the accompanying drawings, in which:

FIG. l is a diagram, partly in block and partly in schematic of oneembodiment of the present invention;

FIG. 2 is a diagram, partly in block and partly in schematic of a secondembodiment of the present invention; and

FlG. 3 is a diagram, partly in block and partly in schematic of a thirdembodiment of the present invention.

Now referring in detail to FIG. l, there is shown cathode-rayoscillograph recording camera 24 which is positioned so as to record thewave form display representative of the input signals on screen 23 ofcathode-ray tube 22. The camera 24 may be such as shown and described inU.S. Patent No. 2,751,275, issued June 19, 1956. Camera 24 may takeeither singly photographs of a particular wave form or continuonsphotographs of a series of wave forms. The lilm in the camera movesatiright angles to the horizontal sweep of the cathode-ray tube, so thatthe result may be a continuous photographing of the display ofsuccessive sweeps and wave forms of input signals visible upon screen 23of cathode-ray tube 22.

In the embodiment of the invention shown in FIG. l, filament 15 isheated to the required temperature for electron emission. A highnegative voltage is applied to terminal 25, and a positive voltage toterminal 36. Terminal 25 is connected to potentiometer 26. Cathode 16 isconnected to point 27 and the voltage thereat is fixed at a slightlyless negative-voltage than available at terminal 25.

Control grid 17 is connected to the variable contact of potentiometer26. This provides variation of control grid voltages and serves as anintensity control of the beam current. Resistor 28 interconnectspotentiometers 26 and 29. The variable contact of potentiometer 29 isconnected to grid 18 and the position of the variable contact controlsthe focusing anode voltage. The focus control varies the anode voltage.Resistor 30 connects potentiometer 29 to ground by way of junction point31. Accelerating anode 19 is kept at a fixed positive value byconnecting it between resistors 34 and 35. Resistor 35 is connected toterminal 36. Vertical deflection plate 20a and horizontal deflectionplate 2lb are connected to accelerating anode 19. The variable contactof potentiometer 38 is connected to vertical deflection plate 20 by wayof resistor 40 and provides vertical centering. The variable contact ofpotentiometer 37 is connected to horizontal deflection plate 21 by wayof resistor 39 and provides horizontal centering. Potentiometers 37 and38 bridge resistors 34 and 35. Resistors 32 and 33 connect resistor 34to ground by way of junction point 31.

Horizontal deflection plate 21 is also connected to the output of timebase generator 41. Time base generator 41 is conventional and producesat its output a saw-tooth voltage which provides the necessaryhorizontal sweep for cathode-ray tube 22. Time Vbase generator 41 isdesigned so as to be synchronized by a pulse provided by way of theoutput of pulse generator 11. Pulse generator 11 is capable ofselectively providing varying repetition frequencies. Terminal receivesthe input signal whose waveform Vis to be displayed on screen 23 ofcathode-ray tube 22.

f An important feature of the present invention resides in thehereinafter described circuitry, wherein terminals 10 and 12 areinterconnected. Terminal 12 is connected to ground by resistor 13 and tocathode 16 by capacitor 14. It is desired to have the input signal onthe semilinear portion of the RC curve so the RC time constant ofresistor 13 and capacitor 14 is selected to be ten times the duration ofthe input signal.

In the operation of the embodiment of the present invention asillustrated in FIG. 1, the intensity of the beam on cathode-ray tube 22is controlled by varying the movable contact on potentiometer 26 so thatthe horizontal trace on screen 23 is just visible. Pulse generator 11 isadjusted to provide a synchronizing pulse having the same pulserepetition frequency as that of the input signal received by way ofinput terminal 10. The sweep sawtooth voltage provided by time basegenerator 41 to horizontal deflection plate 21 is now synchronized withthe pulse from generator 11.

Negative input signals whose waveforms are to be displayed on screen 23and photographed by camera 24 are received at input terminal 10. Fromterminal 10 the input signal passes to vertical deflection plate andalso to terminal 12. It is to be noted that previously the intensity ofthe horizontal trace on screen 23 has been adjusted as to be justvisible. The negative input signals being received by verticaldeflection plate 20 are as a consequence o-f conventional cathode-raytube operation displayed as representative waveforms on screen 23 andwould normally have the same intensity as the horizontal trace. However,the negative input signals are also received at terminal 12. Aspreviously indicated, the RC time constant of resistance 13 andcapacitor 14 is ten times the duration of the negative input signal thusallowing the input signal to be on the semi-linear -portion of the RCcurve. As a result of the negative input signals passing through apreselected RC network, a negative voltage appears on cathode 16 therebydecreasing the bias thereupon during the time the negative input signalsare present. This decrease of bias increases the intensity of the traceonly during negative input periods. It is clear that photographing thewaveforms representative of the negative input signals displayed onscreen 23 by means of camera 24 is greatly improved. The horizontaltrace is just visible and during periods of the negative input signals,the trace is brightened automatically to display on screen 23 thewaveforms representative of said negative input signals. It is also tobe emphasized that the intensity of the horizontal sweep may be adjustedjust below visibility and only the waveforms representative of thenegative input signals will be visible on screen 23 thus eliminatingcompletely the dark background caused by the exposure of the film incamera 24 by the conventionally visible horizontal sweep. The intensitymodulation of the input signal provided by the present invention alsoeliminates the fading out of the leading and trailing edges of signalswhich have a very fast rise time, such as square waves.

Now referring to FIG. 2 which shows a second embodiment of the presentinvention and in structure is identical to that of FIG. l except that anRC network identical to resistor 13 and capacitor 14 of FIG. 1 isconnected to control grid 17 instead of cathode 16. Terminal 42 suppliesthe signal input to the RC network of resistor 43 and capacitor 44. Thetime constant of the RC network of resistor 43 and capacitor 44 is tentimes the duration of the input signal. Thus, it is possible to receivepositive input signals of terminals 10 and 12 and by placing thepositive signal voltage on control grid 17 by way of resistor 43 andcapacitor 44 the bias thereof is decreased, causing greater intensity ofthe trace while a positive input signal is present.

FIG. 3 illustrates a third embodiment of the present invention in whichthe structure thereof is identical to that illustrated in FIG. 1 exceptthat both cathode 16 and control grid 17 have RC networks and the inputt0 each network is comprised of a unidirectional network. The RC networkfor cathode 16 is comprised of resistor 13 and capacitor 14. The RCnetwork for control grid 17 is comprised of resistor 43 and capacitor44. Input terminal 12 is interconnected to the combination of resistor13 and capacitor 14 by diode 45. Input terminal 42 is interconnected tothe combination of resistor 43 and capacitor 44 by diode 46.

In the operation of the embodiment illustrated in FIG. 3, when inputsignals are received by way of terminal 10, it is also received byterminals 12 and 42. The input signals which are negative cause anegative voltage to be received by cathode 17 by way of diode 45 and theRC network consisting of resistor 13 and capacitor 14, thus intensifyingthe trace on screen 23 during the period of said negative input signal.It is to be noted that the negative input signal, however, is not passedby diode 46. In the event that the input signals are positive then apositive voltage is received by control grid 17 by way of diode 46 andthe RC network consisting of resistor 43 and capacitor 44. The positiveinput signals are blocked by diode 45. Thus, it is seen that during thereception of positive input signals the trace on screen 23 isintensified during periods of said positive input signals.

Therefore, the present invention permits the input signal to be used tobias the cathode-ray tube at the identical time it is being used in thevertical deflection of the trace. This allows the photographer to dimout the unimportant sections of the horizontal sweep as he desires `andthus produce better photographs of the input signal.

What is claimed is: j l

An apparatus for displaying waveforms on the screen of a cathode-raytube, said waveforms being representative of negative and positive pulseinput signals comprising means for horizontally sweeping the beam ofsaid cathode-ray tube across said screen, means for synchronizing saidhorizontal sweep to the repetition frequency of said negative andpositive pulse input signals, meansk to adjust the intensity of saidbeam to just below the level of visibility during said horizontal sweep,means to impress said negative and positive pulse input signals upon thevertical deflection plates of said cathode-ray tube, a firstresistor-capacitor network having a preselected time constant connectedto the cathode-ray tube, a second resistor-capacitor network having apreselected time constant connected to the control grid of saidcathode-ray tube, rst unidirectional means to pass only said negativepulse input signals to said rst network simultaneously with theapplication thereof said vertical deection plates, and secondunidirectional means to pass only said positive pulse input signals tosaid `second network References Cited by the Examiner UNITED STATESPATENTS 9/51 Silliman 315-22 12/ 5 2 Fockler 315-22 DAVID G. REDINBAUGH,Primary Examiner.

